Fools' gold found to regulate oxygen

Jul 23, 2012

As sulfur cycles through Earth's atmosphere, oceans and land, it undergoes chemical changes that are often coupled to changes in other such elements as carbon and oxygen. Although this affects the concentration of free oxygen, sulfur has traditionally been portrayed as a secondary factor in regulating atmospheric oxygen, with most of the heavy lifting done by carbon. However, new findings that appeared this week in Science suggest that sulfur's role may have been underestimated.

Drs. Itay Halevy of the Weizmann Institute's Environmental Science and Energy Research Department (Faculty of Chemistry), Shanan Peters of the University of Wisconsin and Woodward Fischer of the California Institute of Technology, were interested in better understanding the global sulfur cycle over the last 550 million years  roughly the period in which oxygen has been at its present atmospheric level of around 20%. They used a database developed and maintained by Peters at the University of Wisconsin, called Macrostrat, which contains detailed information on thousands of rock units in North America and beyond.

The researchers used the database to trace one of the ways in which sulfur exits ocean water into the underlying sediments  the formation of so-called sulfate evaporite minerals. These sulfur-bearing minerals, such as gypsum, settle to the bottom of shallow seas as seawater evaporates. The team found that the formation and burial of sulfate evaporites were highly variable over the last 550 million years, due to changes in shallow sea area, the latitude of ancient continents and sea level. More surprising to Halevy and colleagues was the discovery that only a relatively small fraction of the sulfur cycling through the oceans has exited seawater in this way. Their research showed that the formation and burial of a second sulfur-bearing mineral  pyrite  has apparently been much more important.

Pyrite is an iron-sulfur mineral (also known as fools' gold), which forms when microbes in seafloor sediments use the sulfur dissolved in seawater to digest organic matter. The microbes take up sulfur in the form of sulfate (bound to four oxygen atoms) and release it as sulfide (with no oxygen). Oxygen is released during this process, thus making it a source of oxygen in the air. But because this part of the sulfur cycle was thought be minor in comparison to sulfate evaporite burial, (which does not release oxygen) its effect on oxygen levels was also thought to be unimportant.

In testing various theoretical models of the sulfur cycle against the Macrostrat data, the team realized that the production and burial of pyrite has been much more significant than previously thought, accounting for more than 80% of all sulfur removed from the ocean (rather than the 30-40% in prior estimates). As opposed to the variability they saw for sulfate evaporite burial, pyrite burial has been relatively stable throughout the period. The analysis also revealed that most of the sulfur entering the ocean washed in from the weathering of pyrite exposed on land. In other words, there is a balance between pyrite formation and burial, which releases oxygen, and the weathering of pyrite on land, which consumes it. The implication of these findings is that the sulfur cycle regulates the atmospheric concentration of oxygen more strongly than previously appreciated.

Related Stories

The appearance of oxygen in the Earth's atmosphere probably did not occur as a single event, but as a long series of starts and stops, according to an international team of researchers who investigated rock ...

Today's climate change pales in comparison with what happened as Earth gave birth to its oxygen-containing atmosphere billions of years ago. By analyzing clues contained in rocks, scientists at the Carnegie Institution's ...

(PhysOrg.com) -- Much of our planet's mineral wealth was deposited billions of years ago when Earth's chemical cycles were different from today's. Using geochemical clues from rocks nearly 3 billion years old, a group of ...

Geoengineering -- deliberate manipulation of the Earth's climate to slow or reverse global warming -- has gained a foothold in the climate change discussion. But before effective action can be taken, the Earth's ...

Recommended for you

An analysis of buildings tagged red and yellow by structural engineers after the August 2014 earthquake in Napa links pre-1950 buildings and the underlying sedimentary basin to the greatest shaking damage, ...

As everyone who lives in the San Francisco Bay Area knows, the Earth moves under our feet. But what about the stresses that cause earthquakes? How much is known about them? Until now, our understanding of ...

(Phys.org)—A trio of researchers with the Indian Institute of Science has found, via computer simulation, that deforestation in one part of the world can impact rainfall patterns in another. In their paper ...

It's no surprise that Arctic sea ice is thinning. What is new is just how long, how steadily, and how much it has declined. University of Washington researchers compiled modern and historic measurements to ...

Reasearchers at the University of Cadiz have carried out a study that establishes the atmospheric conditions responsible for the generation of extreme meteorological events in the Gulf of Cadiz, which can ...

User comments : 0

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.

Javascript is currently disabled in your web browser. For full site functionality, it is necessary to enable Javascript.
In order to enable it, please see these instructions.